Featured Links*

You will have to provide. a) Sender b) start & end dates. (timeframe the email was sent to user.)

What I have done so far and suggest you try the same are:-

1- Check rules on the user side if its being filtered/moved to a folder inside inbox. (Mine doesn't)

2- Check junk options. Right click email on user mailbox and go into Junk Options and check blocked senders. (Mine doesn't have the user sending email in that either)

3- Ask the person who is sending email to CC you on it so you can check the msg header. In my the header shows the user who is not getting the email. But message tracker log doesn't even have the person in the receiving field.